The invention relates to a method of determining flow velocities by measuring the phase difference between Doppler signals derived from wave pulses and reflected by substantially the same reflector at two different times, the interval between the times being predetermined, a pair of electric signals being derived according to the method from each Doppler signal by modulation in quadrature, the signals jointly bearing information relating to the phase of the Doppler signal. The invention also relates to a device for performing the method.
In a known method of the aforementioned kind (U.S. Pat. No. 3,914,999), the instantaneous value of the projection of the point velocity in a given direction X is determined from the formula V.sub.x =k.DELTA..phi./.DELTA.t where k is the proportionality constant, .DELTA..phi. is the phase difference between the Doppler signals and .DELTA.t is an integral multiple of the pulse repetition frequency.
In order to improve the signal/noise ratio, the measured results are averaged over a certain time, in which case we have: ##EQU1## where .phi..sub.k is the phase of the Doppler signal after the k.sup.th transmitted pulse, if .DELTA.t=1/PRF (PRF=pulse repetition frequency).
When the velocity profile of a blood flow is transcutaneously measured by the aforementioned method, the problem arises of determining the average phase difference .DELTA..phi. between the Doppler signals with maximum accuracy and in a given time, at a poor signal/noise ratio. (The reason why the signal/noise ratio is poor is easy to understand, since the echoes from blood corpuscles are naturally much weaker than from stationary surrounding structures). To solve this problem it has already been suggested (M. Brandestini, "Topoflow-A Digital Full Range Doppler Velocity Meter", IEEE Transactions on Sonics and Ultrasonics, September 1978, Vl. Su-25, No. 5, pp. 287-293) to use a discriminator comprising a "zero crossing detector" upstream of a sweep integrator. This latter discriminator, which in fact does not determine the average phase difference .DELTA..phi., but an average of the Doppler frequency shift, has the following advantages:
1. The desired linear relation between the phase difference and the output value from the discriminator does not extend over the theoretically possible range of 2.pi.. PA1 2. A decreasing signal/noise ratio results in systematic distortion of the characteristic, resulting in uncontrollable measuring errors in practice. PA1 3. Only scalar values of the phase difference are determined, so that the sign of .DELTA..phi.=.pi. is not defined.